1. Field of the Invention
The present invention relates generally to processes and apparatuses for sterilizing porously surfaced materials, and more particularly, to a method and a device for killing microorganisms on raw meat through controlled applications of a biocidal treatment gas in a vacuum/pressure regulated environment.
2. Description of the Prior Art
Prior to slaughter, the muscles of healthy food animals normally do not contain microorganisms toxic to humans. The animal's gastrointestinal tract, will however harbor several species of microorganisms. Typically, the host of gastrointestinal microorganisms found in animals will include numerous species of bacteria. Enterobacteria such as Salmonella is especially prevalent. While these microorganisms are generally not harmful to the animal, several species will produce toxic reactions in humans.
Unfortunately, an essential part of slaughter is the cutting and removal of an animal's gastrointestinal tract. Not surprisingly, the tract's contents are often spilled and smeared onto the meat surface during and after slaughter. Further contributing to bacterial contamination, gastrointestinal tract contents are often spread from the surface of one animal to another as a result of successive handling by slaughterhouse workers. Contamination can also occur during successive machine-processing steps and when meat pieces are sequentially dipped in various liquid treatment baths. Generally, surface-to-surface contamination can occur throughout every stage in any standard meat slaughtering, processing and packaging assembly-line.
As pointed out in U.S. Pat. No. 3,934,044 (granted to Busch et al. on Jan. 20, 1976) many techniques have been employed in the past for treating meat and meat-cuts to destroy surface bacterial flora. As noted by Busch, however, all of these techniques have suffered from one or more of the following problems: (1) denaturation of meat protein, (2) insufficient bacterial kill, (3) deleterious color change, (4) unacceptable flavor modification, and (5) inadequate control of the process in large scale operations.
The problems enumerated by Busch are evident in several other previous and subsequent patents. For example, U.S. Pat. No. 3,996,386 (granted to Malkki and Nikilla on Dec. 7, 1976) attempts to prevent microbial surface deterioration of foods by spraying the surface of such foods with a preservative in the form of a fine particulate aerosol. As preservatives, Malkki and Nikilla suggest such compounds as chlortetracycline, chloramphenicol, sodium o-phenyl phenolate and others. While Malkki and Nikilla's process provides a means for countering surface bacterial growth, the process still inherently results in a chemically coated food substance, despite Malkki and Nikilla's decided efforts to minimize the actual amounts of preservatives used. Accordingly, potential exists for chemical interactions leading to deleterious color change and/or unacceptable flavor modification.
In the same vein, U.S. Pat. No. 4,636,395 (granted to Robinson, Jr. and Egee on Jan. 13, 1976) describes a method for heat treating the surface area of raw meats by rapidly elevating the temperature of the meat followed immediately by rapid cooling. Although Robinson and Egee do not use taste and/or color modifying preservatives, their process is delicately balanced between the competing objectives of maximizing sufficient bacterial kill, on the one hand, and preventing the denaturation of meat protein, on the other. Having very few safeguards, it is believed that the balance struck by Robinson and Egee is one that can be consistently maintained only with difficulty and constant vigilance. In actual practice, one objective will often have to give way to the other.
As for the Busch process itself, U.S. Pat. No. 3,934,044 indeed attempts to correct the several noted problems. Busch describes a process directed at destroying psychotropic spoilage bacteria on meat or meat-cuts without adversely affecting the color, flavor or aroma of the meat by applying a hot, dilute acid solution to meat surfaces for a brief period of time. However, in the attempt to correct the inherent problems associated with the sterilization of meat products, Busch overlooks a problem that was identified by Malkki and Nikilla in their later patent.
The problem was rooted in the pores, irregularities and other imperfections found on the surface of meat. Such pores and irregularities will often serve as fertile grounds for the development of bacterial growth. Further frustrating the success of conventional sterilization procedures, the removal of surface contamination from meat has proven difficult due also to the strong adhesion of microorganisms to the meat surface. When a wash or spray is used, many organisms tend to escape. While better results were achieved with the addition of bactericides to the spray or wash water, the improvement was slight. Even with the use of concentrated and powerful bactericides, a significant number of organisms still often survived. This occurred even when the exposure time and bactericide concentration were more than adequate to sterilize, a smooth, simple surface.
In accordance with the present invention, the failure of conventional processes may be ascribed to chemical and physical attributes of the solvent, water. Generally, water cannot reach deep contaminated surfaces because of its high surface tension. Many other solvents used for microbial kill exhibit similarly high surface tension. The folds, cavities, and pores are often large enough to contain bacteria; but too small to admit a liquid wash or spray. An impractically high wash pressure would be needed to overcome the capillary pressure in pores having dimensions just large enough to contain bacteria.
Many species of microorganisms are small relative to the surface irregularities in which they live. Many parts of the muscles are about the same size as the target organisms, 2.times.10.sup.-6 m. Poultry muscle fibers, for example, are about 20.times.10.sup.-6 m in diameter; and these each consist of three distinct sheaths, covering many of the much finer fibrils. These fibrils are generally of two kinds: Thick and thin. The thick fibrils are 0.1.times.10.sup.-6 m wide and 2.times.10.sup.-6 m. long. The thin fibrils are 0.05.times.10.sup.-6 m. wide and 0.4.times.10.sup.-6 m. long. In addition, there are several other structures, such as Z discs, mitochondria, capillaries, and cell nuclei in the muscles, capable of obscuring the targets. Furthermore, certain toxic bacteria, such as Salmonella, have short flagella on their surface which may entangle with the meat fibers or fibrils. In view of this, one can appreciate the difficulties inherent in cleaning meat products.
It is in light of the above, that the present invention was designed. In the most general sense, the present invention provides a means to kill microorganisms without damaging the meat product itself. None of patents discussed above, taken either singly or in combination, describe or suggest the instant invention as claimed.